A diplex circuit, or more simply a “diplexer,” is a device which separates or combines RF signals. Diplexers are used in connection with CATV equipment in a number of situations, some of which use two diplexers back-to-back. These include step attenuators, power bypass circuits, cable simulators and equalizer circuits. Many of the prior art CATV diplex circuits are used to act on signals traveling in so-called forward and return paths, e.g., relatively high frequency RF signals pass from a source of such signals to a television set at subscriber premises in the forward direction over one leg of the circuit while lower frequency (DC) signals pass from the premises in the return direction.
Bandstop or “notch” filters are commonly employed in the CATV industry to block transmission of signals in a specified frequency range. For example, certain channels may be designated as premium channels, requiring payment of a fee from the subscriber in order to receive the signals carrying information representing such channels. If the service is not ordered, i.e., the fee is not paid, an appropriate filter is installed in the cable line coming into the non-paying premises. This is but one of the more traditional uses of bandstop filters, i.e., as a so-called trap. A more recent example is the aforementioned use in the handling of forward and return path signals between a head end and subscriber facilities.
Using typical minimum inductor or capacitor design techniques in the lowpass leg of a diplex bandstop filter creates a problem in achieving a flat upper passband, thus causing return loss and increased delay. The capacitor and coil combination that is used to make up the low frequency lowpass filter contains capacitors and coils that are extremely large in value. In fact, the values are large enough to create multiple re-resonances in the upper passband of the highpass leg of the filter, which are undesirable. See, for example, FIG. 1.